Finding Energy Savings With a Biomass Boiler

The story has become increasingly common in the current economic climate: An organization must address its aging infrastructure and building equipment while trying to manage ever-tightening operating budgets. For Arnot Ogden Medical Center, a not-for-profit, 433,000-sq-ft medical facility in Elmira, N.Y., the answer to this challenge came in the form of a renewable-energy technology- and infrastructure-upgrade program designed to positively impact the environment as well as the hospital's bottom line.

Through a $4 million performance contract, wood chips are being turned into energy savings, thanks in part to a biomass boiler system that handles most of the thermal — heating and non-heating — load for the main medical center. The system shifts more than 60 percent of Arnot Ogden's total energy use to a renewable source, stabilizing operating costs and reducing carbon-dioxide (CO2) emissions by more than 9.5 million lb a year, equivalent to removing nearly 970 cars from the road.

Combined with more traditional energy-conservation measures, such as building-envelope improvements and motor upgrades, Arnot Ogden is saving more than $535,000 in annual energy and operational costs. The savings pay for the work and are guaranteed for 10 years. As a result, the improvements do not affect the medical center's operating budget.

Biomass-Powered Heat

Arnot Ogden previously relied on three steam boilers in its central utility plant to meet heating needs. The boilers used natural gas as their primary fuel; however, boilers 2 and 3 also were compatible with No. 2 fuel oil (Table 1).

Equipped with an oxygen- (O2-) trim control system and an economizer for preheating boiler feed water, Boiler 1 was the newest and most efficient boiler and handled 70 percent of the hospital's steam demand. Most of the time, Boiler 2 remained on standby mode when Boiler 1 could meet the hospital's steam load. Boiler 2 operated when Boiler 1 could not meet the hospital's entire steam load, such as during severe weather.

As the oldest and least-efficient boiler, Boiler 3 spent much of its time offline. A 450-hp water-tube boiler, the 35-year-old unit did not have an O2-trim system or a stack economizer and was beyond its useful life.

In 2006, possible measures to address the hospital's aging infrastructure and equipment, including replacing Boiler 3, were discussed. As the boiler neared the end of its useful life, it required more investment through repairs and maintenance — a tough requirement for any hospital operating on a tight budget.

Ideally, the replacement would provide the base load for the hospital, and Boiler 1 would provide the peak load when needed. (Currently, Boiler 1 functions as the primary backup boiler, operating in “warm standby” mode.) Replacing Boiler 3 with a wood-gasifying boiler or a more conventional fossil-fuel natural-gas-fired boiler was considered. A cost analysis was conducted, and the wood gasifier proved economically superior to the fossil-fuel alternative. Although the installation of fossil-fuel-fired equipment would have been less expensive, the wood-gasifying boiler offered a net benefit of nearly $19 million in operational and capital-investment savings over a 20-year life cycle (Table 2).

Spurred by these findings, the hospital replaced the old boiler with a biomass unit capable of producing 15,000 lb of steam per hour at 125 psig. Although the new boiler can be fired with any organic matter that is approximately 2.5 sq in., wood chips and shavings are preferable because they have a fairly low moisture content and are readily available as waste from local mills and lumberyards. A single load of wood-based fuel produces approximately 1 percent of its volume in ash. Each year, Arnot Ogden fuels the biomass boiler with more than 7,000 tons of wood chips from a local mill. The fuel gives the medical center a U.S. Department of Energy-classified carbon-neutral solution for heating its facilities. Table 3 includes calculated CO2 data from the installed system.

The two-chamber boiler-system technology, known as close-coupled gasification, heats wood chips in an airtight, O2-deprived chamber at 1,000°F to 1,400°F until they break down to create a synthetic wood gas — volatile pyrolysis gas — that burns similarly to natural gas (Figure 1). Once released, the gas travels through the burner nozzle where it is superheated at temperatures of up to 2,300°F and mixed with air for complete combustion, leaving little or no waste, such as ash, creosote, or stack effluent. (Any ash residue from the gasifier is non-toxic and suitable for use as compost.) The gas then is treated in a cyclone arrestor to remove any particulate material.

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Also included in the project was a fully automated storage system complete with a 7,200-cu-ft fuel-storage bunker and a material-handling system that feeds the biomass boiler. The underground storage bunker is large enough to meet peak-load plant requirements with a maximum of two fuel deliveries per day. Wood chips are fed to the burner by an automated auger system that is metered to deliver the required fuel based on system load. Therefore, the turndown ratio is almost infinite.

Benefits

From the project's onset, a major priority was to make sure that the improvements made economical and environmental sense for the hospital. The biomass boiler's increased efficiency, safety, and cleanliness helped the project meet those needs. High temperature combustion, a 20-1 turndown ratio, refractory heat storage, and controlled air allow the gasifier not only to respond quickly to boiler demand, but to idle efficiently for economical operation in low-load conditions.

Additionally, natural gas recently has experienced cost swings as high as 35 percent annually. Commercial natural gas averaged $10.50 per million British thermal units, while the wood-chip equivalent averaged $3.90 per million British thermal units based on a fuel cost of $36 per ton. (Projected fuel costs were based on a five-year average for natural gas beginning in 2007.)

The biomass boiler helps insulate the hospital from fuel-price volatility by providing alternative-fuel choices, allowing the hospital to switch energy sources if necessary. Also, the hospital's wood-chip supplier is providing fuel at a fixed price for a six-year period, excluding delivery costs, which are dictated by diesel-fuel cost escalation.

The seasonal efficiency of the biomass boiler (73 percent) is similar to that of a fossil-fuel-fired boiler (75 percent). However, the efficiency of the wood-chip system increases to 78 percent when fuel moisture content reaches 35 percent. Therefore, the average seasonal efficiency of the biomass boiler increases as fuel quality improves, which is fairly simple to achieve.

Limitations

Although Arnot Ogden receives fuel on a regular basis, wood-chip availability could be an issue for other facilities, depending on whether lumberyards and mills are located locally. Further, the biomass system's underground bunker design could be complicated by a high water table. Additionally, the system requires a large amount of floor space to house all of the system components, which can include a high initial price compared with a conventional boiler system.

Meeting Current and Future Needs

Arnot Ogden is planning to construct a new patient tower, and the biomass boiler will help handle the addition to the hospital's heating load efficiently and effectively. The system is sized to provide the hospital with the extra capacity needed to serve the new space when construction is complete. Furthermore, when combined with boilers 1 and 2, the new boiler will provide capacity redundancy, so the entire facility still will receive heat, even if one of the units goes down. Figure 2 illustrates the load and savings potential of the biomass system.

Since its installation during the summer of 2008, the biomass system has helped Arnot Ogden realize approximately $538,000 in energy savings.

A performance-contracting engineer for Honeywell, Michael Daskalakis has more than 20 years of energy-engineering experience and has developed energy-saving performance contracts for commercial and institutional clients throughout the Northeast. He can be reached at michael.daskalakis@honeywell.com. Venkat Iyer is a performance-contracting-engineer manager for Honeywell and has more than 35 years of industry experience. In 2008, he was chairman of the company's performance contracting engineer council; since 2005, he has been an honorary council member. He can be reached at venkat.iyer@honeywell.com.

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